Bug 6725: Make patron duplicate matching flexible

Bug 6725: Make patron duplicate matching flexible

This patch adds a new system preference PatronDuplicateMatchingAddFields
to list the patron's attributes to use for deduplication.
The default value is surname, firstname and dateofbirth to keep existing
behaviour.

Test plan:
0. Apply the patch and execute the update DB entry
1. Create a new patron with surname, firstname
2. Create another patron with the same surname, firstname values
=> Confirm you get the duplicate warning
3. Modify the syspref to edit the list of attributes used to dedup
4. Repeat 1 and 2 with different values and confirm that you get the
behaviours you expect

Note: This is only impacting the add patron form from the UI, not the
import patrons tool.

Signed-off-by: Kelly McElligott <kelly@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>